Electro-optical devices from banana-shaped liquid crystals

ABSTRACT

A liquid crystal device comprising tilted smectic phases of banana-shaped liquid crystal molecules is disclosed. A method for fabricating a light modulating device is also disclosed. The method comprises the steps of providing a pair of substrates with a cell gap therebetween and permanently disposing at least one banana-shaped liquid crystal material into said cell gap. The present invention also provides a method of generating an image, comprising providing a pair of substrates with a cell gap therebetween, providing transparent electrodes on each of the substrates adjacent to the cell gap, disposing at least one banana-shaped liquid crystal material into the cell gap; and applying an electric field across the electrodes. The tilted smectic phases of banana-shaped liquid crystal may be in either a racemic or a chiral state. The application of a sufficiently high electric field transitions the banana-shaped liquid crystal material between the racemic and chiral states, and both the racemic and the chiral states are stable in the absence of an electric field.

This application claims benefit of pending U.S. Provisional ApplicationNo. 60/243,371 filed on Oct. 26, 2000.

GOVERNMENT RIGHTS

The United States Government has a paid-up license in this invention andmay have the right in limited circumstances to require the patent ownerto license others on reasonable terms as provided for by the terms ofGrant DMR8920147-14, awarded by the National Science Foundation.

TECHNICAL FIELD

The present invention resides in the art of electro-optical liquidcrystal devices made with banana-shaped molecules. These devices, whichmay be used for electro-optical switching and electro-optical storageusing liquid crystal devices, utilizes the tilted smectic phase ofbanana-shaped molecules.

BACKGROUND OF THE INVENTION

Liquid crystal materials are materials which occupy an intermediatestate between crystalline solid materials and isotropic liquidmaterials. Liquid crystal materials, while exhibiting an orientationalorder, do not typically exhibit a positional order. The uniqueproperties of liquid crystal materials have enabled their use in avariety of display applications. Among the useful properties of liquidcrystal materials in display applications are the reflection andrefraction of light by the liquid crystal (LC) and the ability of theuser to influence these properties. These properties are governed by theorientation of the molecules which comprise the liquid crystal. Theorientation of individual molecules often determines the behavior oflayers and phases of these molecules.

The lack of mirror symmetry of individual molecules is described as thechirality or “handedness” of the molecule. Many liquid crystal phasesare chiral due to the introduction of chirality of the same sign at themolecular level. Examples of these types of chiral liquid crystal phasesinclude cholesteric, blue, Twist Grain Boundary (TGB) and smectic C*phases. Due to the long-range orientation order of liquid crystallinephases, and the chirality of the molecules, a spontaneous twist occursin a micrometer range. The chirality transfers from a molecular tomesoscopic range, and the phase becomes chiral.

Two molecules that are identical in composition yet are mirror images ofeach other are described as having opposite chirality. This is generallyexpressed as the molecules being left-handed or right-handed dependingon their particular orientation. Liquid crystal molecules having thesame chemical formula but opposite chirality will behave in opticallysimilar, but oppositely directed ways.

Scattering type devices are very well known in liquid crystal displays.Two known types are polymer dispersed liquid crystals (PDLC), andpolymer network containing liquid crystals (PNLC). Liquid crystalpolymer dispersions form a broad class of materials in which the weightconcentration of polymer ranges from 2% to 90%, depending on theapplication and type of polymer used. Dispersions, wherein the liquidcrystal forms nearly spherical droplets randomly distributed throughouta polymer matrix, and the polymer concentration is 20% or more, arenormally referred to as polymer dispersed liquid crystals (PDLC).Normally, PDLCs are light scattering in the “off” state and transparentin the “on” state. It is also possible to make reverse mode PDLCs. Thedisplay modes, however, cannot be interchanged.

PNLCs are formed by photopolymerization of a mixture containing lessthan 10% of a reactive monomer in an aligned liquid crystal host, suchas a nematic, ferroelectric, or cholesteric phase liquid crystalmaterial. The alignment may be assisted by surface alignment layers orby external fields. The polymerization induces phase separation of aninitially homogeneous mixture. The morphology of the polymer networkdepends on the orientational order of the liquid crystal, properties ofthe monomer, and the presence of external aligning fields and/orconventional alignment layers applied to the cell surfaces. Normally,PNLCs work as reverse mode PDLCs. It is also possible to make PNLCs thatare opaque at zero fields. Once made, however, the display modes cannotbe interchanged. The switching times in PDLCs and PNLCs are typicallyover a millisecond, which is not optimal for most video applications.Moreover, the viewing angle and transmittance of the clear state arelimited.

In light of the foregoing, it is evident that there is a need in the artfor an electro-optical liquid crystal device which has faster switchingtimes, a wider viewing angle, and improved transmittance of the clearstate. It would be additionally advantageous if the liquid crystaldevice contained electro-optical storage functionality.

BRIEF SUMMARY OF THE INVENTION

In light of the foregoing, it is a first aspect of the present inventionto provide a liquid crystal display device having faster switchingtimes, a wider viewing angle, and improved transmittance of the clearstate.

It is a further aspect of the present invention to provide a liquidcrystal device capable of electro-optical storage functionality.

The aspects of the invention are achieved by a liquid crystal displaydevice comprising the tilted smectic phase of banana-shaped liquidcrystal molecules. In one embodiment of the invention, a cell isprovided containing the racemic state of banana-shaped liquid crystalmolecules. As used herein, a banana-shaped LC domain is referred to asracemic if the chirality of the layers generally alternates from onelayer to the next. The cell is opaque at zero field, and clear when anelectric field of sufficient magnitude is applied. The clear state isclear in any direction, hence these cells have a very wide viewingangle. This is a tilt separation mode liquid crystal device (TSM-LCD).

In a second embodiment of the invention, a cell is provided containingthe chiral state of the banana-shaped molecules. As used herein withrespect to domains of banana-shaped liquid crystal, the term chiralindicates that adjacent layers of liquid crystal generally have the samechirality or handedness. A cell with a chiral layer arrangement is clearunder zero fields, and becomes opaque when an electric field ofsufficient magnitude is applied. The switching time is more than anorder of magnitude faster than the switching time for PDLCs. This typeof arrangement is used in a chiral separation mode liquid crystal device(CSM-LCD).

In a third embodiment, a cell is provided containing racemic and chiralstate banana-shaped molecules. By applying an electric field, theracemic state is converted to the chiral state, or the chiral state isconverted to the racemic state. Both states are stable at zero field. Inother words, the LC material may be driven to either state with anapplied electric field, and when the field is removed, the state remainsindefinitely. The racemic state is opaque and the chiral state is clear.This cell is suitable as an electro-optical storage device or anelectro-optical switching device.

The present invention also provides a method for fabricating a lightmodulating device, the method comprising the steps of providing a pairof substrates with a cell gap therebetween, and permanently disposing atleast one banana-shaped liquid crystal material into the cell gap.

The present invention also provides a method of generating an image,comprising providing a pair of substrates with a cell gap therebetween,providing transparent electrodes on each of the substrates adjacent tothe cell gap, permanently disposing at least one banana-shaped liquidcrystal material into the cell gap; and applying an electric fieldacross the electrodes to obtain a desired optical state.

These and other aspects of the present invention, as well as theadvantages thereof over existing prior art forms, which will becomeapparent form the description to follow, are accomplished by theimprovements hereinafter described and claimed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a complete understanding of the objects, techniques and structure ofthe invention, reference should be made to the following detaileddescription and accompanying drawings, wherein:

FIG. 1A is the chemical structure of 4-chloro-1,3-phenylenesbis[-4-(4-14alkyloxyphenyliminomethyl)benzoate, a banana-shaped LC with one centralphenyl ring;

FIG. 1B is the chemical structure of3-chloro-3,4′biphenylenebis[4-(tetradecylphenyliminomethyl) benzoate, abanana-shaped LC with a 3,4′-dihydroxybiphenyl central core;

FIG. 2A is a representation of a left hand and a left-handedbanana-shaped LC;

FIG. 2B is a representation of a right hand and a right-handedbanana-shaped LC;

FIG. 3A is a schematic orthogonal view of the racemic B2 phase ofachiral banana-shaped molecules in an antiferroelectric (AFE) state;

FIG. 3B is a schematic orthogonal view of the racemic B2 phase ofachiral banana-shaped molecules in a ferroelectric (FE) state;

FIG. 4A is a schematic orthogonal view of the chiral B2 phase of achiralbanana-shaped molecules in antiferroelectric (AFE) states;

FIG. 4B is a schematic orthogonal view of the chiral B2 phase of achiralbanana-shaped molecules in ferroelectric (FE) states;

FIG. 5 is an enlarged, partial cross-sectional, schematic view of alight modulating device according to the present invention.

FIG. 6 is a graphical representation of the electric field dependence ofthe transmitted light intensity of type 1 cell (d=4-μm, T=70° C., λ=450nm);

FIG. 7 is a photomicrograph of use of the B2 banana phase as anelectrically switchable light shutter, utilizing a 10-μm EHC cell with 1cm² active area of material #1 at room temperature;

FIG. 8 is a photomicrograph of a cell according to the present inventionat room temperature in reflection at zero voltage and at 40V;

FIG. 9A is a photomicrograph of textures of a cell according to thepresent invention in transmission mode;

FIG. 9B is a photomicrograph of textures of a 4-μm thick cell ofmaterial #1 in transmission mode;

FIG. 10 is a graphical representation of the transmission spectra of a 4μm thick cell of material #1;

FIG. 11A is a graphical representation of the voltage dependence of thetime for switching between transparent and scattering states formaterial #1;

FIG. 11B is a graphical representation of the temperature dependence ofthe time for switching between transparent and scattering states formaterial #1;

FIG. 12A is a photomicrograph of the texture of a cell of material B14with 1.5% racemic dopant at 0 V;

FIG. 12B is a photomicrograph of the texture of a cell of material B14with 1.5% racemic dopant at 30 V;

FIG. 12C is a photomicrograph of the texture of a cell of material B14with 1.5% (S)-enantiomer dopant at 0 V; and

FIG. 12D is a photomicrograph of the texture of a cell of material B14with 1.5% (S)-enantiomer dopant at 30 V.

DETAILED DESCRIPTION OF THE INVENTION

Tilted smectic phases of achiral banana-shaped liquid crystal moleculeshave been observed. Banana-shaped or “bent core” liquid crystalmolecules are individually symmetric and therefore have no chiralityindividually. Nonetheless, it has been observed that they can arrange inlayers that exhibit chirality. It is believed that this behavior is dueto spontaneous symmetry breaking which has been observed in tiltedsmectic phases of bent-core, banana-shaped molecules. This phase,sometimes called the B2 phase, is a 2-dimensional fluid. The moleculesadopt a uniform tilt relative to the layer polarization, which isdetermined by the two-fold symmetry axis. Due to the tilt and the polarpacking of the molecules, the layers have no reflection symmetry and aretherefore chiral. The present invention utilizes banana-shaped LCs toconstruct liquid crystal devices. The resulting devices may bemanipulated such that they may be reversibly changed from a lightscattering state to a transparent state and vice versa. These devicesinclude, but are not limited to computer displays, computer monitors,signs, shutters, gratings, optical devices or any other device thattransmits, reflects or modulates light of any wavelength. Thereversibility between states is preferably performed with application ofelectric fields, but could also be accomplished thermally ormechanically.

The banana-shaped LC materials of the present invention may be describedwith reference to FIG. 1A and FIG. 1B, FIG. 1A shows the chemicalstructure of the banana-shaped LC material 4-chloro-1,3-phenylenebis[4-(4-tetradecylphenyliminomethyl) benzoate. FIG. 1B shows the chemicalstructure of 3-chloro-3,4′biphenylenebis[4-(tetradecylphenyliminomethyl)benzoate. As shown in FIGS. 1A and 1B, the molecules have a “bent-core”or “banana-shaped” conformation. Other suitable banana-shaped LCmaterials include those liquid crystals represented by formula I,

R₁, R₂, R₃, R₄ are independently hydrogen or a halogen, and R₅ and R₆are independently C₈-C₁₆ alkyl or C₈-C₁₆ alkoxy. This includes4-chloro-1,3-phenylenebis[4-4-(4-tetradecoxyphenyliminomethyl)benzoates,as well as 1,3-phenylenebis[4-4(4-n-alkylphenyliminomethyl)benzoates,1,3-phenylenebis[4-4(4-n-alkyloxyphenyliminomethyl)benzoates, and1,3-phenylenebis[3-fluroro-(4-n-alkyloxyphenyliminomethyl)benzoates andhalogenated derivatives thereof.

It has been discovered that banana-shaped LC's can exhibit fourdifferent optical states which are antiferroelectric racemic,antiferroelectric chiral, ferroelectric racemic and ferroelectricchiral. As will be discussed in detail, these states are preferablyobtained by applying electric fields of different magnitude and/orfrequency. It is also believed that the magnitude and shape of theapplied electric field—for example, square or triangular—may be used toobtain a desired state. All of these states are obtained without theneed of alignment layers, although the use of alignment materials may bedesirable for some applications.

Prior to discussing each of the four states in detail, it is believedthat the terms used to name the states should be defined and thatproperties that characterize all of the states should also be defined.These properties include: achiral, polar plane, tilt plane, layer,domain, synclinic and anticlinic. Chiral is a term used to describe amolecule or group of molecules, for example, a layer of liquid crystalmolecules, which do not exhibit mirror symmetry. Achiral, on the otherhand, is a term used to describe a molecule or group of molecules whichexhibit mirror symmetry. Banana-shaped liquid crystal molecules aresmectic liquid crystals. That is, they arrange in layers of liquidcrystal molecules with each layer having a particular averageorientational order. In the case of banana-shaped liquid crystals, eachlayer assumes particular polar and tilt directions. The tilt plane of alayer of banana-shaped LC is the plane which shows the tilt of themolecules within the layer relative to the layer normal. The polar planeis the plane which contains the layer normal and the layer polarization.

A group of layers exhibiting a particular pattern of properties isreferred to herein as a domain or phase. A domain may be either racemicor chiral. A racemic domain is one in which the chirality or“handedness” of the layers alternates between left-handed andright-handed from layer to layer. A chiral domain, however, containslayers that have the same chirality.

A domain may also be described according to the tilt direction of thelayers within the domain. A domain is synclinic if all the layers withinthe domain tilt in the same direction relative to the layer normal. Adomain is anticlinic if the direction of the tilt of the layersalternates from one layer to the next.

A domain may also be described by the presence or absence of a netpolarization of the domain. A ferroelectric state is said to exist ifthere is a net polarization of the domain. An antiferroelectric stateexists if the domain exhibits no net polarization.

Further attributes of these states and properties will become apparentas the description proceeds.

Eventhough the banana-shaped LCs of the present invention are achiral,they are capable of assembling to for chiral phases. The orientation ofa left-handed and a right handed banana-shaped LC, relative to the layerin which they are situated, is illustrated in FIG. 2A and FIG. 2B,respectively. P is the layer polarization direction and n is the layernormal. The angle Θ is the angle formed between the average molecularaxis of the layer and the layer normal. As shown in FIG. 2A, the averagemolecular axis of a left handed molecule is oriented clockwise from thelayer normal where the layer polarization is perpendicular to the layernormal. This can be envisioned relative to a left hand as illustrated inFIG. 2A. If the thumb of a left hand is envisioned as pointing in thedirection of layer polarization, the direction of the curling of thefingers represents the direction of the deviation of the molecular axisfrom the layer normal, forming angle Θ. Likewise, in FIG. 2B, theaverage molecular axis of a right handed molecule is orientedcounter-clockwise from the layer normal where the layer polarization isperpendicular to the layer normal. In this case, when the thumb of aright hand is envisioned as pointing in the direction of layerpolarization, the direction of the curling of the fingers represents thedirection of the deviation of the molecular axis from the layer normal,forming angle Θ.

The various arrangements of phases or states of banana-shaped LCmaterials according to the present invention are shown in FIGS. 3A, 3B,4A, and 4B. In these figures, the liquid crystal molecules are shownfrom both the tilt plane view and the polar plane view. The tilt planeview corresponds to the view as seen through a substrate of a liquidcrystal cell according to the present invention while the polar planeview is a view taken from a ninety degree rotation from the tilt planeview. Stated another way, the polar plane contains the layer normal andthe layer polarization (P). The tilt plane is perpendicular to P. Themolecular plane is tilted with respect to the layer normal. The shadingillustrates the orientation of the molecules. The stippled faces of theliquid crystals correspond to the portion of the molecule which is onthe outside of the curve of the molecule while the open faces of theliquid crystals correspond to the portion of the molecule which is onthe inside of the curve of the molecule. “Right” and “Left” designationsin FIGS. 3 and 4 are the chirality descriptors corresponding to right-and left-handed conformations. The single dashed lines representsynclinic interfaces in the anticlinic states. The double dashed linesrepresents defect walls separating oppositely tilted synclinic layers.For each of FIGS. 3A-4B, only two domains containing two layers each areshown for the sake of clarity of the figures. It should be understood,however, that each of the domains may have any number of layers and thatany number of domains may be present within a given liquid crystaldevice according to the present invention.

The layers shown in the figures are right- or left-handed, depending onthe relative orientations of the two-fold symmetry axis and the tiltdirection. The term two-fold symmetry axis is the axis on which themolecule may be rotated 180° with no net change in the structure of themolecule. As mentioned above, the structure is called racemic if thechirality in adjacent layers within a domain alternates, and chiral ifthe adjacent layers within a domain have the same handedness.

FIGS. 3A and 3B show the layer and director structures of a racemic B2banana phase. Most B2 phases have an antiferroelectric (AFE) groundstate. In FIG. 3A, a synclinic tilted smectic (SmC_(S)) polarantiferroelectric (P_(A)) phase is shown. The phase is synclinic, whichmeans that the molecules in adjacent layers within the same domain tiltin the same direction, independent of the chirality of the phases. AnAFE state exists when a phase exhibits no net polarization direction. Inan AFE banana-shaped LC phase, the polarization director alternates 180°from one layer to the next. This arrangement can be seen in the tiltplane view of the two domains shown in FIG. 3A.

The textures of this phase usually consist of fan shaped domains withstripes a few microns wide. Each stripe has a synclinic director tiltstructure with a tilt angle Θ. In the subsequent stripes, the tiltdirections are in the opposite direction. The different tilt directionsbetween one stripe and another are represented by the top and bottomhalves of FIG. 3A. The oppositely tilting synclinic director structuresare separated by defect walls, represented by a double dashed line inFIG. 3A. These defect walls typically have a defractive index differentthan that of the ordered part of the material. The heterogeneity in therefractive index field can lead to a scattering of the unpolarizedlight, making the device opaque.

The AFE racemic state can be switched to a ferroelectric (FE) racemicstate by the application of an electric field below 10 kHz. Aferroelectric state exists in a banana-shaped LC phase when there is anet polarization of a domain. In one embodiment, the change from an AFEto a FE state occurs at a field strength of about less than 10 V/μm. Ithas also been found that by applying a high frequency electric field,i.e. greater than about 10 kHz, the LC material switches from FE racemicto AFE racemic. By applying a low frequency electric field, i.e. below10 kHz, the LC material switches from AFE racemic to FE racemic. FIG. 3Bshows an anticlinic tilted smectic (SmC_(A)) polar ferroelectric (P_(F))phase. The term anticlinic refers to an opposite tilt of the moleculesin adjacent layers within a domain, as seen in the top and bottom partsof the tilt plane view. In this state, the optical axis is parallel tothe layer normal, independent of the sign of the external electricalfield. In the FE state, the defect walls of the AFE state are replacedby synclinic interfaces, which do not scatter light. Thus, a racemicstructure can be switched between a scattering or opaque “off” state byremoving an electric field to a transparent “on” state by applying anelectric field of sufficient magnitude, just as in polymer dispersedliquid crystals (PDLCs). As noted, switch can also be accomplished withchanges in frequency. In contrast to PDLCs, however, the switching timeof a racemic structure from an AFE to FE state is on the order ofapproximately 100 microseconds (μs) or less, which is more than an orderof magnitude faster than the switching time of PDLC devices.

In an FE state, the optical axis is parallel to the layer normal,independent of the sign of the external electrical field. Accordingly,no electro-optical switching is observed when a square wave field isapplied to phases in FE states. That is, there is no variation in theoptical modulation behavior of the LC when the electric field abruptlychanges from negative to positive, or vice versa, with constantamplitude.

The chiral B2 phase is shown in FIGS. 4A and 4B. In FIG. 4A, theanticlinic tilted smectic (SmC_(A)) polar antiferroelectric (P_(A))phase is shown. As described above, the term anticlinic refers to anopposite tilt of the molecules in adjacent layers within a domain, asseen in the top and bottom halves of the tilt plane view. The opticalaxis is again parallel to the layer normal regardless of the handednessof the phases. Layers with different handedness are separated by onlysynclinic interfaces in this phase. Therefore, this state is opticallyclear.

In the FE state of chiral domains, the director structure becomessynclinic. Therefore the phase is described as synclinic tilted smectic(SmC_(S)) polar ferroelectric (P_(F)). In this phase, the left andright-handed synclinic domains are separated by defect walls, whichscatter light. The tilt direction of this phase depends on the sign ofthe electric field. Therefore, electro-optical switching can be observedbetween crossed polarizers when a square wave electric field is appliedto a chiral B2 phase.

As with the above-described racemic phase, the chiral phase can beinduced to change from an AFE state to a FE state by the application ofan electric field. Therefore, a chiral structure may also be switchedfrom a transparent state to an opaque state. As opposed to a racemicstate, however, a chiral state is transparent in the “off” state andscattering or opaque in the “on” state. The switching times between thechiral states are similar to the switching times of the racemic states.

In both the racemic and chiral states, the AFE state may be induced tochange to a FE state by the application of an electric field. During afield-induced AFE to FE transition, the layer chirality is assumed to beconserved. This assumption is generally true for short-term applicationof the fields. In some cases, however, a transformation of the layerchirality can be observed. Stated another way, the SmC_(S)P_(A)arrangement shown in FIG. 3A can be transformed into a SmC_(S)P_(F)shown in FIG. 4A. This transformation from an AFE racemic to an AFEchiral state corresponds to a transformation from a scattering to clearstate. In some materials, the AFE chiral state can then be transformedback to the AFE racemic state by application of triangular-shapedelectric fields. In such an electric field, the field changes frompositive to negative linearly, causing the LC material to becometransiently antiferroelectric as the field passes through a zero fieldstate. In other words, the application of the triangular field wave formto an AFE chiral state transitions the material to a FE chiral statethen an FE racemic state and then to an AFE racemic state. Conversely,application of a square field wave form to an AFE racemic statetransitions the LC material to the FE racemic, to the FE chiral, andthen to the AFE chiral state. Both the antiferroelectric chiral andantiferroelectric racemic states are stable at zero fields. In such anembodiment, a LC device using banana-shaped LCs can be used to form astable image without the continuous application of an electric field. Inother materials, the chiral state spontaneously relaxes back to theracemic state.

Advantageously, the scattering and transparent nature of the variousstates described above render them useful in electro-optical displaydevices. Specifically, the defect walls separating synclinic domains,and their absence in the anticlinic domains, have important consequencesfor display applications, including faster switching times, largerviewing angles, and improved transmittance are possible. Further, thedisplay modes can be interchanged.

Liquid crystal materials suitable for use in the methods and devices ofthe present invention include liquid crystal materials comprisingbanana-shaped molecules. As shown in FIG. 5, a light modulating device10 comprises a pair of opposed substrates 12. Substrates 12 may beglass, plastic or other material commonly known in the art. Transparentelectrodes 14 may be disposed on substrates 12. In one particularembodiment, electrodes 14 are indium-tin oxide. A power source 16, isattached to electrodes 14 through a switch 18. The switch 18 may be usedto connect the power source to the electrodes, to short the electrodes,or to disconnect the electrodes to store charge on them. Operation ofswitch 18, may be controlled by an appropriately designed electronicdrive. Use of an electronic driver circuit allows particular areas of amatrix cell device to be addressed, which in turn allows high contrastbetween the areas. As shown in FIG. 5, a banana-shaped LC material isdisposed between substrates 12 by any known method in the art, such ascapillary action, for example. In FIG. 5, a SmC_(S)P_(F) arrangement isshown, although other arrangements may also be induced as describedherein.

In order to demonstrate the practice of the present invention, thefollowing examples are presented. The specific materials used, the phasesequences of those materials from isotropic to liquid crystal tocrystalline phases and their phase transition temperatures are shown inTable 1.

TABLE 1 Name Composition Phase sequence #1 53% B(4Cl)12 OO + 47% I 130°C. B2 < 20° C. Cr 3FB10q #2 B14 I 153° C. B2 130° C. B3 91° C. Cr #3B14 + 1.5% (1:1 ZLI 811/ZLI I 150° C. B2 127° C. B3 91° C. Cr 3786) #4B14 + 1.5% ZLI 811 I 150° C. B2 127° C. B3 91° C. Cr

The material B(4Cl) 12 OO is4-chloro-1,3-phenylenebis[4-(4-n-oxyphenylpropenoate)benzoate. B(4Cl) 12OO is represented by formula I,

R₁ is chlorine, R₂, R₃, and R₄ are hydrogen, and R₅ and R6 are C₁₂alkoxy. Compound B(4Cl) 12 OO forms a nematic phase in single compoundform.

The material 3FB10q is1,3-phenylenebis[3-fluoro-(4-n-decaoxyphenyliminomethyl)benzoate. 3FB10qis represented by formula I where

R₁ and R₂ are hydrogen, R₃, and R₄ are fluorine, and R₅ and R₆ are C₁₀alkoxy. The single compound forms a so-called B7 banana-phase.

The material B14 is1,3-phenylenebis[4-4(4-tetradecoxyphenyliminomethyl)benzoate. B14 isrepresented by formula I where

R₁, R₂, R₃, and R₄ are hydrogen, and R₅ and R₆ are C₁₄ alkoxy. B14 willform a B2 phase by itself.

ZLI 811 and ZLI 3786 are chiral dopants that are commercially availablefrom Merck. The chemical structures of the two materials are the same,but they are optical antipodes. ZLI 811 has (S) chirality, whereas ZLI3786 has (R) chirality. Materials #3 and #4 have the same structures,but #4 has chiral molecules, whereas #3 has only racemic. Material #1has a B2 phase even at room temperature, and therefore is the mostuseful for practical applications. A 1:1 mixture of materials shown inFIG. 1A and FIG. 1B also forms a B2 phase at room temperature and itsperformance is very similar to material #1. Material #2 was used toillustrate that the disclosed electro-optical mode is not specific forcomposition, but for the B2 phase. Materials #3 and #4 were used toillustrate that the underlying mechanisms require racemic molecules.

The studies were carried out in ready-made cells (4-μm cells fromDisplaytech, 5-μm and 10 μm thick cells from EHC). All cells disclosedwhich are described herein are opposed substrates, either glass orplastic, with electrodes disposed thereon. The cells were filled withthe aforementioned LC material and then sealed. Alignment properties maybe provided on the cells.

Upon cooling from the isotropic phase, samples #1 and #2 formed aracemic phase. The films were opaque because the textures containeddefect walls separating synclinic domains with opposite director tilt.The voltage dependence of the transmitted light intensity through sample#1 is shown in FIG. 6. Circles represent the data points for material #1in increasing fields, while squares represent the data points for thesame material #1 in decreasing fields.

The virgin cell is racemic and moderately scatters the light. Atincreasing electric fields at a frequency of 20 Hz, the transmittedlight intensity increases, especially where it switches to the FE stateat E˜8V/μm field. This behavior can be attributed to the disappearanceof the defect walls separating synclinic domains. At E˜10V/m thetransmittance decreases again, and the film becomes increasingly opaque.This is because the racemic structure becomes chiral. Although notwishing to condition patentability on any particularly theory, it isbelieved that such a transformation can be understood as the preferencefor synclinic interlayer interactions. The transformation to chiraldomains was simultaneously verified by studying the electro-opticalswitching under square wave electric fields with a polarizingmicroscope. At decreasing fields, the field dependence of thetransmittance is monotonous. Transmittance is low at high field, thanincreases as the texture switches back to the AFE state. In subsequentincreasing-decreasing field treatment the material stays in the chiralstate and the latter curve completely reproduces. In other words, in thechiral phase, the material can work as a stable electro-optical devicewithout hysteresis.

The transformation shown in FIG. 6 from the scattering racemic AFE stateto the optically clear, stable, chiral AFE state by high fieldsillustrates the suitability of banana-shaped liquid crystals for use instorage devices.

FIG. 6 also shows the increase in transmittance as the banana-shapedliquid crystals go from the AFE racemic state to the FE racemic state.This characteristic makes them suitable for use in devices wherePDLC-type switching devices were previously used. It should be notedthat in the present example, the FE state does not become completelytransparent, because of the eventual formation of the chiral state.

Another important feature of banana-shaped liquid crystals, shown inFIG. 6, is that the B2 phase can be switched between transparent andscattering states. It is remarkable that at high fields, more than 50%of the light is scattered out. This is similar to a reverse phase PDLC,which has about 60% turbidity, depending on the system.

The capability of using the banana-shaped liquid crystals displays aslight shutters is illustrated in FIGS. 7A and 7B. Behind a cell filledwith Material # 1, a sheet of paper is placed with the name “ALCOM”written on it. At zero field the film is transparent and the text isvisible as seen in FIG. 7A. At fields E>8V/μm, the film is opaque andthe text is not visible (FIG. 7B). It is important to note that thesituation does not depend on the viewing angle. This is a majoradvantage over PDLCs.

The cells were also measured in reflection by microscope withoutpolarizers under laser illumination. The corresponding cells are shownin FIGS. 8A and 8B which are photomicrographs of 4 μm cells of material#1 at room temperature. FIG. 8A shows a border area of the cell at zerovoltage. FIG. 8B is the same cell with an applied voltage of 40V. Theimages represent 60 μm×60 μm areas.

The textures of cells containing banana-shaped LCs in transmission withwhite light illumination are shown in FIGS. 9 and 9B. FIG. 9 is a pairof photomicrographs showing textures of a 4 μm thick cell of material #1in transmission mode without polarizers. The temperature was 23° C. Thearea shown is a 500 μm×300 μm area at the edge of the electrode. Thephotomicrograph on the left shows the transmission properties of thecell with no electric field. The photomicrograph on the right shows thetransmission properties of the cell with an electric field applied. FIG.9B is a pair of photomicrographs of the cells shown in FIG. 9, exceptthe photomicrographs shown in FIG. 9B are at a higher magnification.Each photomicrograph shows a 50 μm×40 μm area of the cells.

The wavelength dependence of the scattering effect is shown in FIG. 10.The transmission spectra shown is for a 4 μm thick cell at material #1at 80° C.

The voltage and the temperature dependencies of the switching times of aLC according to the present invention are shown in FIG. 11. FIG. 11A isa graph showing switching time versus voltage for a 4 μm cell containingmaterial #1 at 75° C. FIG. 11A shows that the switching times aregenerally 600 μs or less. When V/4 μm equals about 90, the switchingtime is about 100 μs. FIG. 11B shows a graph showing switching timeversus temperature for a 4 μm cell containing material #1 at a voltageof 70V. It is seen that even as far as 70° C. below the clearing point,the switching time is below 100-μs. This is more than an order ofmagnitude faster than the switching time of PDLC devices.

Similar results, with somewhat weaker scattering is observed in cellscontaining materials #2 and #3. Transmission in the scattering state is60% of the clear state. In addition, using these materials the chiralstate can be transformed back to the racemic state by changing theelectric field waveform shape from square-wave to triangular. Thetransitions take about 1 second with a frequency of about 1 kHz.

The scattering almost disappears in cells containing material #4, whichcontains chiral dopant. The transmittances in OFF and ON states differonly by 5%. This clearly proves that the scattering is connected to thepresence of left- and right-handed domains. The 1.5% chiral dopant makesthe material almost completely uniformly chiral. The differences betweenthe racemic and chiral textures are presented in FIG. 12. In each ofFIGS. 12A-12D, the figure is a photomicrograph of the texture of a100-μm×70-μm area of a 4-μm cell between crossed polarizers at atemperature of 123° C. FIG. 12A is a photomicrographic representation ofthe texture of a of material B14 with 1.5% racemic dopant, at 0V. FIG.12B is a photomicrograph of the texture of material B14 with 1.5%racemic dopant at 30V. FIG. 12C is a photomicrograph of the texture ofmaterial B14 with 1.5% (S)enantiomer dopant, at 0V. FIG. 12D is aphotomicrograph of the texture of material B14 with 1.5% (S)-enantiomerdopant, at 30V. It can be seen that the racemic material breaks up intosmall domains in the ferroelectric state. The domains of the chiralmaterials are similar and substantially without defect lines in both theferroelectric and the antiferroelectric state.

In one preferred embodiment of the present invention, a liquid crystalcell is provided which contains banana-shaped liquid crystal moleculesin the racemic state. The switching takes place between synclinic andanticlinic structures at zero and sufficiently high (E>E_(th)) A.C.electric fields are applied. In appearance the film is opaque at zerofield and clear under electric fields. The scattering at low fields isdue to the defects separating oppositely tilted synclinic domains in theantiferroelectric state. Under sufficiently strong fields aferroelectric state is induced where the defect walls disappear, becausethe oppositely tilted synclinic domains are anticlinic. The clear stateis clear in any direction. As the scattering is based on the tiltseparation, it is called a tilt separation mode liquid crystal device(TSM-LCD).

In a second preferred embodiment, a cell is provided which containsbanana-shaped liquid crystal molecules in the chiral state.Electro-optical switching takes place between anticlinic and synclinicstructures as zero and sufficiently high (E>E_(th)) A.C. electric fieldsapplied. In appearance the film is clear at zero field and opaque underelectric fields. At low fields the structure is antiferroelectric wherethere are no defect walls but only synclinic interfaces in an anticlinicbackground, therefore no light scattering appears. The scattering athigh fields is due to the defects separating oppositely tilted synclinicdomains in the ferroelectric state. Due to the overall racemic nature ofthe molecules the texture splits to left and right-handed synclinicdomains separated by walls that scatter the light. Because the lightscattering in this case is caused by defect walls that separate chiraldomains, it can be called a chiral separation mode liquid crystal device(CSM-LCD).

In a third preferred embodiment, a cell is provided that contains bothracemic and chiral banana-shaped structures. Application of asufficiently high electric field produces reversible transitions betweenthe racemic and chiral structures. Both states are stable at zerofields. The racemic state is scattering and is obtained by applicationof a triangular shape form and the chiral state is optically clear andis obtained by application of a rectangular shape form. These devicesare suitable for optical storage devices. As this method is based ontransitions between racemic and chiral states, it is called aracemic-chiral transitions mode liquid crystal device (RCT-LCD).

Although in appearance the disclosed methods and displays are similar toPDLC-s and PNLC-s, the underlying principles are completely different.There are important differences in the performances of the devices ofthe present invention and the PDLCs and PNLCs of the prior art. InPDLC-s and PNLC-s, scattering is due to heterogeneous materials. Theyinvolve the coexistence of solid and liquid crystal phases. In thepresent case, however, there is only one phase having either differentdirector tilt, or opposite chiral handedness.

In PDLC-s and PNLC-s the switching times are over one millisecond,whereas in the devices of the present invention switching times can be10-μs or less. This is about two orders of magnitude faster than PDLCsand PNLCs. This is due to the polar nature of these phases, whichprovide first order interactions between field and polarization. Inaddition, the viewing angle and the transmittance of the clear state arenot limited when banana materials are used.

The liquid crystal devices of the present invention have commercialapplication possibilities in all the areas where PDLC-s are currentlyused. This includes privacy windows, projectors, and the like. Inaddition, because the performance of the display devices of the presentinvention is superior in several aspects, including larger viewing angleand faster switching, the application possibilities are broader. Thefact that the racemic and chiral states work in opposite fashion and canbe exchanged reversibly implies, for example, use in a privacy window.Such a window does not use any energy, except during switching from onestate to other. A RCT-LCD could also be used in electronic newspapers,or in other optical data storage devices. The time for transformationfrom one state to the other requires about a second, which is about thetime to turn one page over in a book. Accordingly, they are completelysatisfactory for these applications. In addition, a display can beswitched to a mode, in which it stays in the chiral state and would beswitched at a video rate for viewing motion pictures. This capabilitywould make it useful in cellular phones, laptops or palmtops, etc. Theyalso can be used in guest-host type displays with dichroic dyes.Furthermore, it is envisioned that they could be used in one and twodimensional switchable gratings for beam steering, and as opticalswitches for information technology. During the transformation betweenracemic and chiral states, any state is stable, enabling multistablestorage devices with gray scale properties. Gray scale may be achievedby varying the voltage magnitude or alternatively, by varying thefrequency of the electric field.

Thus, it can be seen that the objects of the invention have beensatisfied by the structure and its method for use presented above. Whilein accordance with the Patent Statutes, only the best mode and preferredembodiment has been presented and described in detail, it is to beunderstood that the invention is not limited thereto or thereby.Accordingly, for an appreciation of true scope and breadth of theinvention, reference should be made to the following claims.

1. A liquid crystal cell comprising: a banana-shaped liquid crystalmaterial in a racemic state, wherein said cell can be reversiblyswitched between clear and opaque states, wherein said liquid crystalmaterial is in an anti-ferroelectric state and said cell is opaque atzero applied electric field and further wherein said liquid crystalmaterial switches to a ferroelectric state and said cell becomes clearupon application of an electric field to said cell.
 2. A liquid crystalcell according to claim 1, wherein defect walls separate synclinicdomains in said liquid crystal material when said liquid crystalmaterial is in an anti-ferroelectric state, and wherein said defectwalls are replaced by synclinic interfaces when said liquid crystalmaterial is in a ferroelectric state.
 3. A liquid crystal cell accordingto claim 2, wherein said defect walls have a refractive index differentfrom that of an ordered part of said liquid crystal material, resultingin a non-homogenous refractive index field for said cell when saidliquid crystal material is in said anti-ferroelectric state.
 4. A liquidcrystal cell according to claim 1, wherein a polarization director ofsaid liquid crystal material when in said anti-ferroelectric statealternates 180° between adjacent layers of said material.
 5. A liquidcrystal cell according to claim 1, wherein a switching time between saidanti-ferroelectric and ferroelectric states is approximately 100 μs orless.
 6. A liquid crystal cell according to claim 1, wherein said liquidcrystal material will switch from the anti-ferroelectric to theferroelectric state upon application of an electric field of less thanabout 10 kHz and wherein said liquid crystal material will switch fromthe ferroelectric to the anti-ferroelectric state upon application of anelectric field of greater than about 10 kHz.
 7. A liquid crystal cellaccording to claim 1, further comprising a pair of opposed substrateswith electrodes disposed thereon.
 8. A liquid crystal cell according toclaim 1, wherein the banana-shaped liquid crystal material is selectedfrom the group consisting of compounds represented by the formula:

where R₁, R₂, R₃, and R₄ are independently hydrogen or a halogen, and R₅and R₆ are independently C₈-C₁₆ alkyl or C₈-C₁₆ alkoxy.
 9. A liquidcrystal cell according to claim 1, wherein the banana-shaped liquidcrystal molecules are selected from4-chloro-1,3-phenylenebis[4-(4-14alkyloxyphenyliminomethyl)benzoates];1,3-phenylenebis[4-4(4-n-alkyphenyliminomethyl)benzoates];1,3-phenylenebis[4-4(4-(4-n-alkyloxyphenyliminomethyl)benzoates];1,3-phenylenebis[3-fluoro-(4-n-alkyloxyphenyliminomethyl)benzoates]; andhalogenated derivatives thereof.
 10. A liquid crystal cell comprising: abanana-shaped liquid crystal material in a chiral state, wherein saidcell can be reversibly switched between clear and opaque states, whereinsaid liquid crystal material is in an anti-ferroelectric state and saidcell is clear at zero applied electric field and further wherein saidliquid crystal material switches to a ferroelectric state and said cellbecomes opaque upon application of an electric field to said cell.
 11. Aliquid crystal cell according to claim 10, wherein synclinic interfacesseparate synclinic domains in said liquid crystal material when saidliquid crystal material is in an anti-ferroelectric state, and whereinsaid synclinic interfaces are replaced by defect walls when said liquidcrystal material is in a ferroelectric state.
 12. A liquid crystal cellaccording to claim 11, wherein said defect walls have a refractive indexdifferent from that of an ordered part of said liquid crystal material,resulting in a non-homogenous refractive index field for said cell whensaid liquid crystal material is in said ferroelectric state.
 13. Aliquid crystal cell according to claim 10, wherein a polarizationdirector of said liquid crystal material when in said ferroelectricstate alternates 180° between adjacent layers of said material.
 14. Aliquid crystal cell according to claim 10, wherein a switching timebetween said anti-ferroelectric and ferroelectric states isapproximately 100 μs or less.
 15. A liquid crystal cell according toclaim 10, wherein said liquid crystal material will switch from theanti-ferroelectric to the ferroelectric state upon application of anelectric field of less than about 10 kHz and wherein said liquid crystalmaterial will switch from the ferroelectric to the anti-ferroelectricstate upon application of an electric field of greater than about 10kHz.
 16. A liquid crystal cell according to claim 10, wherein thebanana-shaped liquid crystal material is selected from the groupconsisting of compounds represented by the formula:

where R₁, R₂, R₃, and R₄ are independently hydrogen or a halogen, and R₅and R₆ are independently C₈-C₁₆ alkyl or C₈-C₁₆ alkoxy.
 17. A liquidcrystal cell according to claim 10, wherein the banana-shaped liquidcrystal molecules are selected from4-chloro-1,3-phenylenebis[4-(4-14alkyloxyphenyliminomethyl)benzoates];1,3-phenylenebis[4-4(4-n-alkyphenyliminomethyl)benzoates];1,3-phenylenebis[4-4(4-(4-n-alkyloxyphenyliminomethyl)benzoates];1,3-phenylenebis[3-fluoro-(4-n-alkyloxyphenyliminomethyl)benzoates]; andhalogenated derivatives thereof.
 18. A liquid crystal cell according toclaim 10, further comprising a pair of opposed substrates withelectrodes disposed thereon.
 19. A liquid crystal cell comprising: abanana-shaped liquid crystal material present in both racemic and chiralstates disposed between said substrates, wherein said cell can bereversibly switched between opaque and clear states by the applicationof an electric field.
 20. A liquid crystal according to claim 19,wherein said racemic state is opaque, said chiral state is clear, andsaid switching between opaque and clear states is accomplished byreversible transitions between the racemic and chiral structures.
 21. Aliquid crystal according to claim 19, wherein said racemic state isobtained by application of a triangular wave shaped electric field andwherein said chiral state is obtained by the application of arectangular wave shaped electric field.
 22. An electro-optical switchingdevice comprising a liquid crystal cell according to claim 1.